This invention pertains to a method for rejuvenating cracking catalysts. The invention particularly concerns a method for rejuvenating a zeolitic crystalline aluminosilicate cracking catalyst which has been deactivated by cyclic cracking and regeneration.
Catalytic cracking systems employ catalysts in a moving bed or a fluidized bed. The catalytic cracking operation is carried out in the absence of externally supplied molecular hydrogen and is, for that reason, distinctly different from hydrocracking, in which molecular hydrogen is added in processing. In catalytic cracking an inventory of particulate catalyst is continuously cycled between a cracking reactor and a catalyst regenerator. In a fluidized catalytic cracking (FCC) system, a stream of hydrocarbon feed is contacted with fluidized catalyst particles in the hydrocarbon cracking zone, or reactor, at a temperature of about 425.degree.-600.degree. C., usually 460.degree.-560.degree. C. The reactions of hydrocarbons at the elevated operating temperature result in deposition of carbonaceous coke on the catalyst particles. The resulting fluid products are separated from the deactivated, spent catalyst and are withdrawn from the cracking reactor. The spent catalyst particles, containing a substantial concentration of coke, are stripped of volatiles, usually by means of steam, and are then passed to the catalyst regenerator. In the regeneration zone, the coked catalyst is contacted with a predetermined amount of molecular oxygen. A desired portion of the coke is burned off the catalyst, simultaneously restoring catalyst activity and heating the catalyst to a higher temperature required for use in the cracking zone, e.g., 540.degree.-815.degree. C., usually 590.degree.-730.degree. C. Flue gas is formed by combustion of the coke in the regenerator. The flue gas may be treated for removal of particulates and conversion of carbon monoxide, after which it is normally discharged into the atmosphere.
The activity of a cracking catalyst is an important parameter in catalytic cracking operations. A standard measure of the activity of a cracking catalyst is the degree of conversion which can be obtained using the catalyst in a cracking operation. The degree of conversion of feed hydrocarbons obtained in a cracking operation may be defined as the volume percent of fresh hydrocarbon feed having a normal boiling point of at least 221.degree. C. which is changed to gasoline and lighter hydrocarbon products during the cracking conversion step, where the end point of gasoline for the purpose of determining conversion may be defined as 220.degree. C. In addition to being a measure of catalyst activity, the conversion obtained with the catalyst can also be used as a measure of the severity of a cracking operation, so the activity of a catalyst is determined by conversion at a predetermined, standard set of operating conditions. At a given set of operating conditions, a more active catalyst gives a greater conversion than does a less active catalyst. Increased conversion is a desirable attribute in a cracking catalyst. Higher conversion allows flexible operation of a cracking unit. For example, when conversion is raised, feed throughput can be increased, or a higher degree of feed conversion can be maintained with a constant feed throughput.
Because of catalyst attrition, imperfect gas-solids separation, etc., catalyst in a cracking unit is continuously being lost from the circulating inventory. The desired catalyst inventory level is conventionally maintained by constant addition of fresh, or rejuvenated, make-up catalyst. Accordingly, the catalyst inventory in a given FCC unit is a mixture of particles which have been in use for widely varying periods, and which contain varying amounts of coke and contaminants such as metals. The mixture of catalyst particles forming the inventory during normal unit operation is referred to as equilibrium catalyst. The activity of catalyst used in commercial cracking units is generally measured on the basis of the average activity of the equilibrium catalyst. Fresh cracking catalyst is known to possess a much higher activity than equilibrium catalyst or catalyst which has been used in a cracking operation for a relatively short time.
The use of zeolitic crystalline aluminosilicates, or molecular sieves, as the primary, active components in cracking catalysts is well known. Most commercial zeolite-type catalysts contain a molecular sieve of the faujasite crystal structure. Zeolite Y-type molecular sieves are especially favored for use in cracking catalysts. In manufacturing zeolite cracking catalysts, the zeolite component, as synthesized, is subjected to ion-exchange in order to activate and stabilize the zeolite prior to its catalytic use. This ion-exchange procedure typically substitutes protons, proton precursors such as ammonium, or rare earth cations for the sodium cations present in the zeolite as synthesized. The X-type and Y-type zeolites used commonly for cracking catalysts have a faujasite-type crystal structure and have a silica/alumina molar ratio of about 3 to 5. Other zeolites, such as mordenite and ZSM-5-type crystalline aluminosilicates, which have different characteristic crystal structures and have silica/alumina molar ratios above 6, have also been proposed for use in cracking catalysts. In commercial cracking catalysts, the zeolite component is composited with a matrix or binder precursor such as clay or a silica-alumina hydrogen, which facilitates shaping the final catalyst. The matrix material may or may not have some catalytic cracking activity of its own.
Various procedures have been suggested for treating used, deactivated cracking catalysts, such as equilibrium catalyst, in order to increase their activities. It is known in the art that nickel, iron and vanadium which are present in hydrocarbon feeds used in cracking build up on the catalyst and tend to reduce the activity of the catalyst. The contaminant metals can also reduce the product selectivity obtained. Schemes for removing metals from used catalyst have been developed to overcome this problem. For example, U.S. Pat. No. 3,168,481 discloses a process for removing vanadium from cracking catalysts by leaching the catalysts with ammonia. U.S. Pat. No. 3,684,738 discloses acid treatment as a method for treating zeolites which have a high silica/alumina molar ratio in order to activate the zeolite for use in catalytic cracking.
Rejuvenation of zeolite-containing hydrocracking catalysts by various procedures such as aqueous acid or base treatment have been suggested. Hydrocracking catalysts typically include an active metal component, such as a Group VIII metal, which must be highly dispersed on the catalyst matrix to provide optimum catalyst activity. Since the hydrogenation activity of the active metal component in hydrocracking catalysts is of major importance, regeneration schemes used in treating hydrocracking catalysts are normally directed primarily at achieving redispersion of the active metal. For example, U.S. Pat. No. 3,835,028 discloses a method for rejuvenating a zeolite-containing hydrocracking catalyst by an aqueous treatment with an ammonium salt, and U.S. Pat. No. 4,055,482 discloses rejuvenation of a zeolite-containing hydrocracking catalyst by aqueous treatment with an acid.